1. Field of the Invention
This invention relates to a radiation image processing apparatus, wherein signal processing for determining the shape and location of an irradiation field, adjusting read-out conditions for a final readout from a preliminary read-out image signal, adjusting image processing conditions, and/or detecting an abnormal pattern is carried out on an image signal representing a radiation image by using a neural network. This invention also relates to a determination apparatus, which is provided with a neural network having a learning function by a back propagation method and which determines a certain item. This invention further relates to a radiation image read-out apparatus for reading out a radiation image from a recording medium, such as a stimulable phosphor sheet, on which the radiation image of an object has been recorded, and thereby obtaining an image signal representing the radiation image.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In order for an image signal to be detected accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. Novel radiation image recording and reproducing systems which accurately detect an image signal have been proposed. The proposed radiation image recording and reproducing systems are constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out in order approximately to ascertain the radiation image stored on the stimulable phosphor sheet. In the preliminary readout, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary readout is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out to obtain the image signal, which is to be used during the reproduction of a visible image. In the final readout, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image readout and the output of a read-out means. For example, the term "read-out conditions" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or to the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The term "image processing conditions" as used herein means a group of various factors, which are adjustable and set when an image signal is subjected to processing, which affect the gradation, sensitivity, or the like, of a visible image reproduced from the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
As disclosed in, for example, U.S. Pat. No. 4,638,162 and Japanese Unexamined Patent Publication No. 61(1986)-280163, operations for calculating the values of the read-out conditions for the final readout and/or the image processing conditions are carried out by a group of algorithms which analyze an image signal (or a preliminary read-out image signal). A large number of image signals detected from a large number of radiation calculate the read-out conditions for the final readout and/or the image processing conditions are designed on the basis of the results obtained from this processing.
In general, the algorithms which have heretofore been employed are designed such that a probability density function of an image signal is created, and characteristic values are found from the probability density function. The characteristic values include, for example, the maximum value of the image signal, the minimum value of the image signal, or the value of the image signal at which the probability density function is maximum, i.e. the value which occurs most frequently. The read-out conditions for the final readout and/or the image processing conditions are determined on the basis of the characteristic values.
Recently, a method for utilizing a neural network, which is quite different from the algorithms described above, has been proposed.
The neural network is provided with a learning function by back propagation method. Specifically, when information (an instructor signal), which represents whether an output signal obtained when an input signal is given is or is not correct, is fed into the neural network, the weight of connections between units in the neural network (i.e. the weight of synapse connections) is corrected. By repeating the learning of the neural network, the probability that a correct answer will be obtained in response to a new input signal can be kept high. (Such functions are described in, for example, "Learning representations by back-propagating errors" by D. E. Rumelhart, G. E. Hinton and R. J. Williams, Nature, 323-9,533-356, 1986a; "Back-propagation" by Hideki Aso, Computrol, No. 24, pp. 53-60; and "Neural Computer" by Kazuyuki Aihara, the publishing bureau of Tokyo Denki University).
The neural network is also applicable when the read-out conditions for the final readout and/or the image processing conditions are to be adjusted. By feeding an image signal, or the like, into the neural network, outputs representing the values of the read-out conditions for the final readout and/or the image processing conditions can be obtained from the neural network.
Also, recently, in the radiation image recording and reproducing systems which use X-ray film or stimulable phosphor sheets, particularly in such radiation image recording and reproducing systems designed to facilitate medical diagnoses, not only have image signals been processed in ways which ensure that the visible images produced from them will be of high quality, but image signals have also been processed in ways which allow certain image patterns to be extracted from radiation images. One type of processing which results in extraction of an image pattern is disclosed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-125481.
Specifically, an image pattern can be detected in a complicated radiation image by processing the image signal representing it in various ways. The image signal is made up of a series of image signal components, and with appropriate processing the image signal components corresponding to a particular image pattern can be found. For example, from a very complicated radiation image, such as an X-ray image of the chest of a human body, which includes various linear and circular patterns, a pattern corresponding to a tumor, or the like, can be detected.
After a pattern, for example, a tumor pattern, is detected in a complicated radiation image, such as an X-ray image of the chest of a human body, a visible image is reproduced and displayed such that the detected pattern can be viewed clearly. Such a visible image can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
When the neural network is utilized to adjust the read-out conditions for the final readout and/or the image processing conditions, by repeating the learning of the neural network, the read-out conditions for the final readout and/or the image processing conditions appropriate for a specific radiation image can be determined. However, in a single system for processing X-ray images of, for example, the shoulder of a human body, various types of image signals are obtained which represent various radiation images, such as the images of the right shoulder and the left shoulder (reversed images), an enlarged image and a reduced image, an erect image and a side image and an inverted image, and images shifted from each other. In order for a neural network to be constructed which can determine the read-out conditions for the final readout and/or the image processing conditions appropriate for each of various such images, a very large number of units should be incorporated in the neural network. Also, a storage means should be used which has a very large capacity for storing information representing the weight of connections between units in the neural network. Additionally, the learning of the neural network should be repeated very many times.
In view of the above circumstances, in U.S. patent application Ser. No. 687,140, the applicant proposed an apparatus for adjusting read-out conditions and/or image processing conditions for a radiation image wherein, even if various image signals representing various radiation images are obtained, the read-out conditions for the final readout and/or the image processing conditions appropriate for each of the various radiation images are determined by a neural network provided with a comparatively small number of units.
Specifically, as an apparatus which is applicable when a stimulable phosphor sheet is used and the preliminary readout is carried out, the applicant proposed an apparatus for adjusting read-out conditions and/or image processing conditions for a radiation image, wherein a first image signal representing a radiation image of an object is obtained by exposing a stimulable phosphor sheet, on which the radiation image has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, the emitted light being detected,
a second image signal representing the radiation image is thereafter obtained by again exposing the stimulable phosphor sheet to stimulating rays, the light emitted by the stimulable phosphor sheet being detected, and
read-out conditions, under which the second image signal is to be obtained, and/or image processing conditions, under which the second image signal having been obtained is to be image processed, are adjusted on the basis of the first image signal,
the apparatus for adjusting read-out conditions and/or image processing conditions for a radiation image comprising:
i) a storage means for storing information representing a standard pattern of radiation images, PA1 ii) a signal transforming means for transforming said first image signal representing said radiation image into a transformed image signal representing the radiation image, which has been transformed into said standard pattern, and PA1 iii) a condition adjusting means provided with a neural network, which receives said transformed image signal and feeds out information representing the read-out conditions and/or the image processing conditions. PA1 i) a storage means for storing information representing a standard pattern of radiation images, PA1 ii) a signal transforming means for transforming said image signal representing said radiation image into a transformed image signal representing the radiation image, which has been transformed into said standard pattern, and PA1 iii) a condition adjusting means provided with a neural network, which receives said transformed image signal and feeds out information representing the image processing conditions. PA1 i) a storage means for storing information representing a standard pattern of radiation images, PA1 ii) a signal transforming means for transforming said first image signal representing said radiation image into a transformed image signal representing the radiation image, which has been transformed into said standard pattern, PA1 iii) a condition adjusting means provided with a neural network, which receives said transformed image signal and feeds out information representing the read-out conditions and/or the image processing conditions, PA1 iv) a display means for displaying the radiation image, which has been read out under the read-out conditions fed out of said condition adjusting means and/or image-processed under the image processing conditions fed out of said condition adjusting means, as a visible image, and PA1 v) an input means for entering correcting information, which is used to transform said visible image displayed on said display means into a more appropriate visible image, into said condition adjusting means, PA1 i) a storage means for storing information representing a standard pattern of radiation images, PA1 ii) a signal transforming means for transforming said image signal representing said radiation image into a transformed image signal representing the radiation image, which has been transformed into said standard pattern, PA1 iii) a condition adjusting means provided with a neural network, which receives said transformed image signal and feeds out information representing the image processing conditions, PA1 iv) a display means for displaying the radiation image, which has been image processed under the image processing conditions fed out of said condition adjusting means, as a visible image, and PA1 v) an input means for entering correcting information, which is used to transform said visible image displayed on said display means into a more appropriate visible image, into said condition adjusting means, PA1 i) a dictionary means for storing a plurality of sets of input signals and output signals, which represents correct answers corresponding to the input signals, for a plurality of modes, PA1 ii) a retrieval means for retrieving a set, which is associated with an input signal having a high level of correlation with an input signal fed into said neural network, from said dictionary means, and PA1 iii) a learning means for carrying out learning operations of said neural network by utilizing the input signal and the output signal, which constitute said set retrieved by said retrieval means, as an input signal and an instructor signal, respectively, of said neural network. PA1 i) a preliminary read-out means for exposing a stimulable phosphor sheet, on which the radiation image has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, detecting the emitted, and thereby obtaining a preliminary read-out image signal representing said radiation image of said object, PA1 ii) a final read-out means for again exposing said stimulable phosphor sheet to stimulating rays, which cause said stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, detecting the emitted, and thereby obtaining a final read-out image signal representing said radiation image of said object, PA1 iii) an operation means provided with a neural network, which receives information based on said preliminary read-out image signal and feeds out information representing read-out conditions, under which said final read-out image signal is to be obtained, and/or image processing conditions, under which said final read-out image signal having been obtained is to be image processed, PA1 iv) a condition input means for manually entering information representing read-out conditions and/or image processing conditions, and PA1 v) a retraining means for carrying out retraining of said neural network by utilizing the information representing the read-out conditions and/or the image processing conditions, which information has been received from said condition input means, as an instructor signal, and utilizing the information based on said preliminary read-out image signal, which information corresponds to said read-out conditions and/or said image processing conditions, as an input signal. PA1 i) a read-out means for reading out a radiation image of an object from a recording media, on which the radiation image has been recorded, and thereby obtaining an image signal representing said radiation image of said object, PA1 ii) an operation means provided with a neural network, which receives information based on said image signal and feeds out information representing image processing conditions, under which said image signal is to be image processed, PA1 iii) a condition input means for manually entering information representing image processing conditions, and PA1 iv) a retraining means for carrying out retraining of said neural network by utilizing the information representing the image processing conditions, which has been received from said condition input means, as an instructor signal, and utilizing the information based on said image signal, which information corresponds to said image processing conditions, as an input signal. PA1 i) a preliminary read-out means for exposing a stimulable phosphor sheet, on which the radiation image has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, detecting the emitted, and thereby obtaining a preliminary read-out image signal representing said radiation image of said object, PA1 ii) a final read-out means for again exposing said stimulable phosphor sheet to stimulating rays, which cause said stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, detecting the emitted, and thereby obtaining a final read-out image signal representing said radiation image of said object, PA1 iii) an operation means for determining read-out conditions, under which said final read-out image signal is to be obtained, and/or image processing conditions, under which said final read-out image signal having been obtained is to be image processed, the determination being carried out from said preliminary read-out image signal, PA1 iv) a judgment means for judging whether the read-out conditions and/or the image processing conditions, which have been determined by said operation means, are or are not correct, and PA1 v) an input means for entering information representing the correctness or incorrectness of the read-out conditions and/or the image processing conditions, which have been determined by said operation means, PA1 i) a read-out means for reading out a radiation image of an object from a recording media, on which the radiation image has been recorded, and thereby obtaining an image signal representing said radiation image of said object, PA1 ii) an operation means for determining image processing conditions, under which said image signal is to be image processed, from said image signal, PA1 iii) a judgment means for judging whether the image processing conditions, which have been determined by said operation means, are or are not correct, and PA1 iv) an input means for entering information representing the correctness or incorrectness of the image processing conditions, which have been determined by said operation means, PA1 i) a preliminary read-out means for exposing a stimulable phosphor sheet, on which the radiation image has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, detecting the emitted, and thereby obtaining a preliminary read-out image signal representing said radiation image of said object, PA1 ii) a final read-out means for again exposing said stimulable phosphor sheet to stimulating rays, which cause said stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, detecting the emitted, and thereby obtaining a final read-out image signal representing said radiation image of said object, PA1 iii) an operation means for determining read-out conditions, under which said final read-out image signal is to be obtained, and/or image processing conditions, under which said final read-out image signal having been obtained is to be image processed, the determination being carried out from said preliminary read-out image signal, PA1 iv) a correction means for calculating correction values for the read-out conditions and/or the image processing conditions, which have been determined by said operation means, and PA1 v) an input means for entering information representing the correction values for the read-out conditions and/or the image processing conditions, PA1 i) a read-out means for reading out a radiation image of an object from a recording media, on which the radiation image has been recorded, and thereby obtaining an image signal representing said radiation image of said object, PA1 ii) an operation means for determining image processing conditions, under which said image signal is to be image processed, from said image signal, PA1 iii) a correction means for calculating correction values for the image processing conditions, which have been determined by said operation means, and PA1 iv) an input means for entering information representing the correction values for the image processing conditions,
Also, as an apparatus which is applicable when a stimulable phosphor sheet and other recording media are used and the image processing conditions are adjusted, the applicant proposed an apparatus for adjusting image processing conditions for a radiation image, wherein image processing conditions, under which an image signal is to be image processed, are adjusted on the basis of the image signal representing a radiation image of an object,
the apparatus for adjusting image processing conditions for a radiation image comprising:
However, radiation images of, for example, human bodies have very complicated configurations. Therefore, particularly for radiation images to be used in making diagnoses, it often occurs that users, e.g. physicians, want to correct the read-out conditions for the final readout and/or the image processing conditions, which have been determined by the neural network after its learning operations have been carried out. In such cases, after the neural network is located in a hospital, or the like, it is desired that re-learning operations of the neural network be carried out.
As for abnormal pattern detecting apparatuses, Japanese Unexamined Patent Publication No. 62(1987)-25481 discloses an apparatus wherein an image signal representing an X-ray image of the chest of a human body, or the like, is processed with a specific filter, which does not change with positions in the X-ray image, and a circular pattern and a linear pattern are thereby detected. The detected circular pattern is displayed as a prospective tumor pattern, and the detected linear pattern is displayed as a blood vessel pattern.
However, radiation images of human bodies have very complicated configurations. For example, a tumor pattern appearing in close proximity to a rib pattern and a tumor pattern appearing at an intermediate position between two rib patterns in an X-ray image of the chest of a human body will be different from each other. Therefore, with the aforesaid conventional abnormal pattern detecting apparatus having a simple configuration, the problem occurs in that all of tumor patterns, which are present in an X-ray image, cannot be found accurately. Also, the problem occurs in that a pattern, which does not actually correspond to a tumor, is found by mistake as a tumor pattern. After an image pattern is detected and a visible image showing the detected image pattern is reproduced in, for example, a radiation image recording and reproducing system designed to facilitate medical diagnoses, a physician will base his diagnosis primarily on how the detected pattern looks. If a certain pattern (a certain tumor pattern) is not detected accurately, a physician may fail to find a tumor. This is a very serious problem in making diagnoses.
In order for the aforesaid problem to be eliminated, the filter, which is used to process an image signal representing a radiation image, or the like, may be designed such that all of patterns, which are at least considered as being prospective tumor patterns, can be detected. However, if all of patterns, which are at least considered as being prospective tumor patterns, are detected, patterns (noise) which do not actually correspond to tumors will also be detected as tumor patterns. Therefore, the reliability of the automatic image finding systems becomes bad, and the efficiency of diagnoses cannot be kept high.
Heretofore, in cases where no system for automatically finding the images is available, physicians, who make diagnoses from X-ray images of the chests of human bodies, which images are recorded on, for example, sheets of X-ray film, have considerably accurately detected tumor patterns, from their knowledge and experience, even when the tumor patterns are present at various positions in an X-ray image and are slightly deformed.
Also, even if the same types of image forming and viewing systems are used, it will often occur that the mean-level characteristics of the images to be processed vary for different locations of the image forming and viewing systems, or the like. It will also occur that the results of a judgment made from an image vary slightly for different persons who view the image. For example, when X-ray image recording and diagnostic systems for X-ray images of the chests of human bodies are used, the energy distribution and the dose of X-rays employed in image recording operations vary for different hospitals, in which the X-ray image recording and diagnostic systems are located. In such cases, slightly different X-ray images of the chests will be formed in different hospitals. Also, when a diagnosis is made from a reproduced visible X-ray image of the chest, the results of the judgment as to whether the chest is normal or is to be re-examined will vary for different physicians who view the X-ray image.
Therefore, in the systems for automatically finding the images, by utilizing knowledge and experience, a higher level of processing than the simple filtering processing of an image signal representing an image should be carried out such that patterns which do not actually correspond to tumors, or the like, may be eliminated as much as possible from the patterns, which have been found at least as being prospective tumor patterns, or the like. Also, by accumulating the knowledge in accordance with the locations of the systems for automatically finding the images and persons who view the images, the systems for automatically finding the images should be modified through re-learning operations such that the systems become more suitable for the users.
Methods for carrying out an operation (hereinafter referred to as an EDR) for calculating the values of the read-out conditions for the final readout and/or the image processing conditions from an image signal (or a preliminary read-out image signal) have the problems described below. Specifically, the EDR is carried out automatically by a group of algorithms which analyze the image signal. The selection of which algorithms are appropriate for a specific image signal depends on the characteristics of the recorded image. Such characteristics include, for example, what portion of an object is represented by the recorded image (e.g., the head, the chest or the abdomen in cases where the object is a human body) and what mode was used when the image was recorded (e.g., an ordinary image recording mode, a contrasted image recording mode or an enlarged image recording mode). A large number of image signals detected from a large number of radiation images are statistically processed and classified according to the characteristics of the recorded images, such as those characteristics mentioned above. The algorithms which calculate the read-out conditions for the final readout and/or the image processing conditions are designed on the basis of the results obtained from this processing.
However, because the algorithms selected for an EDR are designed on the basis of the results of the statistical processing of a large number of image signals as described above, the algorithms cannot be appropriate for all radiation images, even though they are selected on the basis of specific characteristics of a recorded image. In cases where an unsuitable EDR is carried out on an image signal, a visible image having a density and latitude which make it unsuitable for viewing purposes is obtained when the visible image is reproduced from the image signal detected from the radiation image. In the worst case, a visible image which cannot provide the necessary information about a radiation image is obtained, and the image must be rerecorded. Also, in cases where the object is a human body, the radiation dose to the human body is doubled when the recording of the image is repeated. This problem should be avoided because radiation is harmful to the human body.
Examples of cases where the aforesaid problems arise will be described hereinbelow.
One of the characteristics of a recorded image which should be considered when selecting the algorithms for an EDR is that unnecessary portions of an object may be recorded on a recording medium when scattered radiation impinges upon those portions. Also, radiation may impinge directly upon a portion of a recording medium without being passed through or reflected by an object. In this manner, an image signal picks up unnecessary components which must be removed in order for an image signal representing only the desired portions of a radiation image to be obtained.
FIGS. 13A and 13B are graphs showing probability density functions of preliminary read-out image signals SP detected by preliminary readouts carried out-on two stimulable phosphor sheets.
FIG. 13A shows an example of the probability density function of a preliminary read-out image signal SP detected from a radiation image for which an EDR is suitable and which is of the type accounting for most (for example, 99.5%) radiation images.
With reference to FIG. 13A, the values of the preliminary read-out image signal SP, which were obtained by detecting the light emitted by a stimulable phosphor sheet during a preliminary readout and which are proportional to the amount of light emitted, are plotted on the horizontal axis, which has a logarithmic scale. The relative frequency of occurrence of the values of the preliminary read-out image signal SP is plotted on the vertical axis at the upper part of the graph, and the values of the image signal obtained during the final readout are plotted on a logarithmic scale on the vertical axis at the lower part of the graph. In this case, the probability density function of the preliminary read-out image signal SP is composed of projecting parts A, B, and C, and it is assumed that the projecting part B corresponds to the part of a radiation image which it is necessary to reproduce. By way of example, in order for the projecting part B to be found, values of the probability density function are compared to a predetermined threshold value T, starting with the value of the function at the minimum value SL of the preliminary read-out image signal SP and working along the direction of increase of the image signal values, i.e. along the chained line. When the probability density function crosses through the threshold value T, a calculation is made to find out whether the function is rising or falling. In this manner, a second rising point "a" and a second falling point "b" are found. The maximum and minimum values of the preliminary read-out image signal at the points "b" and "a" are denoted by Smax and Smin, respectively. The read-out conditions for the final readout are set so that during the final readout the image information represented by the emitted light signal for values of the emitted light falling within the range of Smax to Smin is reproduced accurately. Specifically, the read-out conditions for the final readout are set such that Smax and Smin of the preliminary read-out image signal SP are detected respectively as the maximum image signal value Qmax and the minimum image signal value Qmin in the final readout. The maximum image signal value Qmax and the minimum image signal value Qmin in turn correspond respectively to the maximum density Dmax and the minimum density Dmin within the predetermined correct density range of the visible image ultimately reproduced. More specifically, the read-out conditions for the final readout are set such that during the final readout the image information represented by values of the emitted light signal falling within the range of Smax to Smin is detected as an image signal with values lying on the straight line G shown in FIG. 13A.
In the manner described above, for most of the radiation images, the read-out conditions for the final readout can be adjusted appropriately. However, in some cases, the correct read-out conditions for the final readout cannot be determined with this method. One such case will be described hereinbelow.
FIG. 13B shows the probability density function of a preliminary read-out image signal SP' detected from a radiation image of an object approximate to the object, the radiation image of which yielded the probability density function shown in FIG. 13A. In the case of both FIGS. 13A and 13B, the radiation image of the objects (by way of example, the chest of a human body) were recorded under the same image recording conditions, i.e. the characteristics of the recorded images were the same.
When the probability density function shown in FIG. 13B is compared with that shown in FIG. 13A, projecting parts B' and C' approximate the projecting parts B and C, respectively. However, a projecting part A' differs from the projecting part A, in that it is divided into two projecting parts, A1' and A2'.
When the method described above is applied to the probability density function shown in FIG. 13B, the values of the probability density function are compared to the predetermined threshold value T. Starting from the values of the probability density function corresponding to the minimum value SL' of the preliminary read-out image signal SP', whenever the value of the probability density function crosses over the threshold value T, a calculation is made as to whether the function is rising or falling. In this manner, a second rising point a' and a second falling point b' are found. However, the range of the preliminary read-out image signal SP' between the points a' and b' is different and far apart from the range (of the projecting part B') corresponding to the part of the radiation image, which it is necessary to reproduce. If the final readout is carried out so that during the final readout the image information represented by an emitted light signal with values falling within the range between the points a' and b' is detected as an image signal with values lying on a straight line G', the image signal thus obtained will not contain the necessary image information, and cannot yield a useful visible image. In such cases, the recording of the radiation image of the object must be repeated.
Besides the extreme case described above, an inaccurate EDR deteriorates the image quality of a reproduced visible image.
FIG. 13C shows the probability density function of a preliminary read-out image signal SP' detected from a radiation image of an object approximate to the object, the radiation image of which yielded the probability density function shown in FIG. 13A. In the case of both FIGS. 13A and 13C, the radiation image of the objects (by way of example, the chest of a human body) were recorded under the same image recording conditions, i.e. the characteristics of the recorded images were the same.
When the probability density function shown in FIG. 13C is compared with that shown in FIG. 13A, projecting parts A' and C' approximate the projecting parts A and C, respectively. However, a projecting part B' differs from the projecting part B, in that it is divided into two projecting parts, B1' and B2'.
When the method described above is applied to the probability density function shown in FIG. 13C, the values of the probability density function are compared to the predetermined threshold value T. Starting from the values of the probability density function corresponding to the minimum value SL' of the preliminary read-out image signal SP', whenever the value of the probability density function crosses over the threshold value T, a calculation is made as to whether the function is rising or falling. In this manner, a second rising point a' and a second falling point b' are found. However, the range of the preliminary read-out image signal SP' between the points a' and b' is different from the range (of the projecting part B' composed of the projecting parts B1' and B2') corresponding to the part of the radiation image, which it is necessary to reproduce. If the final readout is carried out so that during the final readout the image information represented by an emitted light signal with values falling within the range between the points a' and b' is detected as an image signal with values lying on a straight line G', the image signal thus obtained will not contain part of the necessary image information, and cannot yield a visible image representing all of the necessary image information. In such cases, it often occurs that the recording of the radiation image of the object must be repeated.
Even if a visible image representing almost all of the necessary image information can be obtained, the problems will occur in that the image density of the visible image becomes undesirably low or high as a whole or the brightness of the visible image becomes low.
In U.S. Pat. No. 4,928,011, a novel method has been proposed to eliminate the aforesaid problems regarding an automatic EDR. With the proposed method, the read-out conditions for the final readout and/or the image processing conditions are determined automatically by the EDR, and a means is provided for manually setting or adjusting the read-out conditions for the final readout and/or the image processing conditions. By way of example, judgment criteria are predetermined, and a judgment is made as to whether the read-out conditions for the final readout and/or the image processing conditions, which have been determined by the EDR, are or are not correct. When it has been judged with the judgment criteria or by an operator that the determined read-out conditions for the final readout and/or the determined image processing conditions are not correct, correct read-out conditions for the final readout and/or correct image processing conditions are set manually. Alternatively, the read-out conditions for the final readout and/or the image processing conditions, which have been determined by the EDR, are corrected manually.
An EDR employed in a radiation image recording and reproducing system has been designed by the manufacturer of the radiation image recording and reproducing system on the basis of the results of the statistical processing of a large number of image signals detected from a large number of radiation images. However, it often occurs that a specific user uses the radiation image recording and reproducing system under specific conditions different from those anticipated by the manufacturer. For example, a certain radiation image recording and reproducing system for processing radiation images of human bodies as objects is designed in the manufacturing step such that images of the chests of human bodies are primarily processed, and the mean-level error in automatically determining the read-out conditions for the final readout and/or the image processing conditions with respect to images portions of human bodies other than the chests is larger than error in automatically determining the conditions with respect to images of the chests. However, it often occurs that a specific user processes only radiation images of the heads. Also, it often occurs that a specific user employs a radiation image recording apparatus having a large level of nonuniformity in how radiation is irradiated, and therefore large errors occur in determining read-out conditions for the final readout and/or the image processing conditions by the standard algorithms. In such cases, incorrect read-out conditions for the final readout and/or incorrect image processing conditions are frequently obtained with the automatic EDR, and therefore manual adjustments of the read-out conditions for the final readout and/or the image processing conditions must be carried out frequently. Accordingly, the radiation image recording and reproducing system becomes unsuitable for the specific user.
Also, in a radiation image recording and reproducing system designed such that a judgment is made as to whether the read-out conditions for the final readout and/or the image processing conditions, which have been automatically determined with an EDR, are or are not correct, and the conditions are manually adjusted when they are judged as being incorrect, the conditions may be judged as being correct when the values of the conditions fall within the range of, e.g. the mean value .+-. a predetermined value. The conditions may be judged as being incorrect when the values of the conditions go beyond this range. However, a radiation image of an object fluctuates largely in accordance with the object and the conditions under which the radiation image was recorded. Therefore, an accurate judgment cannot always be achieved with the aforesaid judgment criteria. For example, it often occurs that the read-out conditions for the final readout and/or the image processing conditions, which have been judged with the judgment criteria as being correct, are actually incorrect, or those which have been judged with the judgment criteria as being incorrect, are actually correct. One method for eliminating such problems is to select a small value as the aforesaid predetermined value such that incorrect conditions may not be judged as being correct and all of the values of the conditions, which at least has the probability of being incorrect, may be judged as being incorrect. However, with such a method, considerable time and labor are required to adjust the conditions manually, and the operating efficiency of the radiation image recording and reproducing system cannot be kept high.